In recent years, research and development have been actively carried out regarding white light-emitting devices using blue light-emitting elements as light-emitting sources. In particular, the demand for white light-emitting diodes using blue light-emitting diode elements has been expected to be rapidly expanded from now on because the diodes are light in weight and long in lifetime, without using mercury. It is to be noted that a light-emitting device using a light-emitting diode element as a light-emitting element is referred to as a light-emitting diode (LED). The method adopted most commonly as a method for converting blue light from a blue light-emitting diode element to white light is intended to obtain white light in a pseudo-manner by mixing blue light with yellow light that has a complementary relationship with the blue light. As described in, for example, Patent Literature 1, a white light-emitting diode can be configured in a way that a coating layer containing a phosphor that absorbs some of blue light and emits yellow light is provided over the entire surface of a diode element that emits blue light, and a molded layer or the like is arranged ahead thereof for mixing blue light from the light source with yellow light from the phosphor. A YAG (Y3Al5O12) powder activated with cerium (which may be hereinafter referred to as YAG: Ce in some cases) or the like is used as the phosphor.
In the case of a white light-emitting device using a blue light-emitting diode element and a YAG: Ce phosphor, light from a blue light-emitting diode element commonly used today has a blue light with a peak wavelength around 460 nm (for example, CIE 1391 chromaticity coordinates (which may be hereinafter referred to simply as chromaticity in some cases) Cx=0.135 and Cy=0.08). This is because the luminescent efficiency of the YAG: Ce phosphor is increased in this wavelength range. On the other hand, the color from a phosphor of YAG: Ce with an emission wavelength unadjusted (which may be hereinafter referred to as an unadjusted YAG: Ce in some cases) is a yellow color with a peak wavelength around 530 to 545 nm (for example, chromaticity Cx=0.41 and Cy=0.56). For this reason, when light around 460 nm from the blue light-emitting diode element is mixed in color with light from the unadjusted YAG: Ce phosphor, the obtained light will deviate from white light (for example, 6000 K: chromaticity Cx=0.32 and Cy=0.34) toward a green color. Therefore, in order to obtain a white color from this configuration, there is a need to use a YAG: Ce phosphor with a fluorescence peak wavelength on the further red side (longer wavelength side).
In addition, since white light-emitting diodes vary in required chromaticity range (color temperature), depending on the intended use such as displays, lights, and backlight sources, it is necessary to select the phosphor also in response to the intended use. Furthermore, in order to stabilize the chromaticity of the LED, it is desirable to use one type of phosphor, rather than the concurrent use of a plurality of phosphors. Therefore, in the case of a YAG: Ce phosphor that has a broad fluorescence spectrum, it is essential for the fluorescence dominant wavelength to be set within the required range, as a standard of the emission wavelength. Typically, adjustments to the emission wavelengths are carried out by shifting the peak wavelengths of phosphor materials to the longer wavelength side or shorter wavelength side.
As a method for shifting the peak wavelength of a phosphor material, for example, it is known as a known technique that the peak at a fluorescence wavelength of a YAG: Ce phosphor can be shifted on the order of 10 nm to the longer wavelength side by increasing or decreasing the amount of Ce as an activation agent (Non Patent Literature 1). In addition, it is known as a known technique that the peak of a fluorescence wavelength is shifted to the longer wavelength side by partially substituting an element Y with an element Gd (Non Patent Literatures 2 and 3). Patent Literature 1 proposes a method for obtaining a white color (chromaticity Cx=0.33 and Cy=0.33), in a way that the YAG: Ce phosphor with the fluorescence wavelength adjusted to the longer wavelength side in this way is combined with a blue light-emitting diode element to constitute a white light-emitting diode.
On the other hand, the inventors have proposed a ceramic composite for light conversion, which includes a solidified body formed so that a plurality of oxide phases including a YAG: Ce fluorescent phase and an Al2O3 phase are continuously and three-dimensionally entangled mutually, and a white light-emitting device including a combination of a blue light-emitting element and the ceramic composite for light conversion (Patent Literature 2). The ceramic composite for light conversion can stably achieve homogeneous yellow fluorescence because the YAG: Ce fluorescent phase is uniformly distributed, and has excellent heat resistance because of being made of ceramic. Furthermore, because the Al2O3 phase is a main constituent phase of oxide, the composite has advantages such as better heat conduction than that of a resin dispersion system and resistance to ultraviolet light. In addition, the configuration of a white light-emitting device requires no resin, because the composite per se is a bulk body. For this reason, white light-emitting devices using the ceramic composite for light conversion vary insignificantly, and are extremely preferred for achieving higher output.
Also as for the ceramic composite for light conversion as described in Patent Literature 2, the inventors have demonstrated that the peak at a fluorescence wavelength can be adjusted in the range of 550 to 560 nm or 540 to 580 nm by adjusting the entire composition of the solidified body as described in Patent Literature 3. However, in the case of a ceramic composite for light conversion, which is obtained by the unidirectional solidification method described in Patent Literature 3, it has been found that when the proportion of Gd or Ce is increased to adjust the fluorescence wavelength to the longer wavelength side, phases other than the YAG: Ce fluorescent phase and YAG: (Gd, Ce) fluorescent phase as well as the Al2O3 phase may be produced to decrease the value (total radiant flux) of spectral integral of obtained white light in some cases.
In order to deal with this problem, Patent Literature 4 suppresses the production of phases other than the YAG: Ce fluorescent phase and YAG: (Gd, Ce) fluorescent phase as well as the Al2O3 phase, thereby making it possible to obtain a ceramic composite for light conversion, which maintains a high radiant flux even when the peak at a fluorescence wavelength of the ceramic composite for light conversion is adjusted to the longer wavelength side.